During the industrial large-scale production of modules with electronic components, normally a large number of temperature sensitive components must be soldered to corresponding contact pads on a circuit board. For this purpose the electronic components comprise appropriate solder surfaces, which for conventional components are provided as wire connections or contact pins, or which are provided in the form of metal surfaces at appropriate points, which is the case for components for surface mounting (SMD components). With the on-going miniaturisation of modules the individual components are becoming continually smaller, wherein the type of the component and its purpose in part imply a certain minimum size. For example, resistors, diodes and small-signal transistors are manufactured in extremely small cases, whereas other components, such as inductors, capacitors or power transistors are accommodated in significantly larger cases. Since increasingly, all types of devices are being electronically controlled, normally a large number of components with small-signal properties as well as a range of components with higher power or larger volume requirement is necessary. An appropriately designed module therefore normally comprises components with very different sizes and shapes and consequently with very different thermal behaviour.
For a perfect mechanical and electrical joint between a component or its metal connection surface, and a corresponding contact pad on the board, liquefaction of the solder paste for no longer than approximately 60 seconds is required, wherein however the individual components may be subjected to a specified temperature depending on the component specification for only a very limited time. The liquidus temperature of typical solder pastes lies in the region of 183 to 227° C., corresponding to a temperature which for most of the components used is not critical for a range of many minutes. The subjection of a module to a temperature just above the liquidus temperature would therefore avoid the risk of overheating small components which quickly become hot, but would lead to a longer soldering phase, because the metal surfaces on the small components would have already exceeded the liquidus temperature, whereas the metal surfaces on large components would not yet allow any liquefaction of the solder paste. The very long soldering for the small components arising in this case generally leads however to a defective soldering result and so this technique appears to be less attractive. A slow and therefore uniform heating of all components up to just below the liquidus temperature of the solder paste could significantly reduce the problem mentioned above, but in practice there are problems because certain activators in the solder paste lose their function before the actual solder process and in addition, an unwanted oxidation of the metal surfaces to be soldered occurs, wherein the wetting properties of the solder contacts with liquid solder is impaired.
In many known devices the item to be soldered is heated in a preheating zone to a temperature in the range from 150 to 160° C. (when using solders containing lead) or 160 to 200° C. (when using lead-free solders), wherein the heating occurs such that essentially a temperature equilibrium can form in the item to be soldered. Following that, the item to be soldered is brought into a soldering zone in which it is subjected to a significantly higher temperature, so that initially the components are brought to a temperature above the liquidus temperature of the solder paste and the solder process occurs during the liquefaction of the solder paste. Since the heating and the solder process are to take place in a time frame of a maximum of 50 to 60 seconds, generally high temperatures of 240 to 300° C. are used in the soldering zone. As previously mentioned, generally small components assume a high temperature more quickly than correspondingly inert and large components, so that with these high temperatures prevailing in the soldering zone the danger of overheating small components arises, which can result in malfunctions or the premature failure of the components and therefore of the complete module. Consequently, attempts are being made on one hand to facilitate rapid heating of the components above the liquidus temperature, wherein on the other hand however the risk of overheating smaller components is reduced. In this connection the German patent specification DE 197 41 192 describes a reflow soldering method for soldering an item to be soldered in a transit oven, wherein the item to be soldered is brought to a temperature below the melting temperature of the solder in a preheating zone by means of preheating devices and is then soldered in a soldering zone by means of heating equipment and is then cooled in a cooling zone to a temperature below the melt temperature. The soldering method described in DE 197 41 192 is characterised in that the item to be soldered is first brought into a first region of the soldering zone in which it is subjected by means of a first convector heater to a temperature which is significantly above the melt temperature of the solder. Then the item to be soldered is brought into a following second section of the soldering zone in which the item to be soldered is subjected by means of a second convector heater to a lower temperature which however still lies above the melting temperature of the solder. Here, in this described reflow soldering method the initially high first temperature is to enable heating in particular of the smaller components above the liquidus temperature, wherein in the following step with lower temperature overheating of the smaller components is to be avoided, whereas the larger components continue to increase their temperature up to the required liquidus temperature.
Although the previously mentioned method can lead to a more balanced run of temperature when soldering large and small components, the exact adjustment of the first and second temperatures and the corresponding dwell time in the first section with the high temperature must be matched to the respective module, because otherwise there is the risk that small components will be overheated in the first high temperature stage. When processing a large number of different modules, the extent to which the machine can be loaded is restricted, because the temperature adjustment for the various modules must be readjusted or there is the risk of an increased failure rate due to the overheating of small components if the same setting is used for different modules.